When assembled a ball on the end of the stroke rod slides in a ball housing similar to that of a tubular jar with a knuckle movement ability. The top sub has a threaded pin connection with a fish neck and the bottom sub has a threaded box. Knuckle jars come in three and five inch strokes, creating an effective jarring action with the added functionality of a knuckle joint.

Operation

Knuckle jars replace mechanical jars when the lubricator length is not sufficient to except a wireline tool string with mechanical jars.

Knuckle jars are limited to running such things as static gradients tests or extended tests where the recorders are hung at run depth, or in deviated wells.

Precautions

Do not run knuckle jars in any situation where it is known that jarring will be part of the operation.